Actins are major components of all eukaryotic cells. Functionally they are involved in the cytoskeletal structures and these structures are altered in transformed cells. To understand the structure and expression of the human actin genes, we have begun a molecular analysis of molecularly cloned human actin genes. DNA sequence analysis of the human smooth muscle actin gene (aortic type) showed that this gene has a unique intron not found in any other actin genes analyzed, suggesting that this intron was inserted late in the evolutionary process. The smooth muscle actin gene in chemically transformed human cells showed a point mutation not found in the normal cell counterpart. To study whether the cloned human cardiac actin DNA contained sequences necessary for its expression, a recombinant plasmid was constructed by inserting the cardiac actin DNA into the expression shuttle vector, bovine papilloma viral DNA. Mouse cells transformed with this recombinant expressed a substantial amount of the human cardiac mRNA and this mRNA was faithfully translated into cardiac actin in mouse cells. The results indicate that the cloned actin DNA contains (within its 5' and 3' untranslated sequences) signals for the initiation and termination of transcription as well as for translation. Bovine papilooma viral DNA vectors can be used to stably express cloned human DNA sequences in mouse cells.